High-power semiconductor laser diodes are often designed with a wide electrically pumped cavity, which supports several lateral spatial modes. Such cavities or waveguides are termed multi-mode. Because the waveguide width is larger than the width of a single mode fiber or slab waveguide, coupling the laser output efficiently into a single mode waveguide presents a basic problem.
Many prior art methods of extracting high single-mode power out of inherently multimode structures, such as passive coherent combination, rely on self-organizing nonlinear behaviors of coupled oscillators, for example.
While U.S. Pat. No. 6,944,192 in the name of Prassas assigned to Corning Incorporated and U.S. Pat. No. 6,580,850 in the name of Kazarinov assigned to Applied WDM Inc. show devices which appear to be similar, they both use the principle of adiabatic transformation of the beam profile from the laser. An adiabatic transformation produces a single mode output by utilizing only one mode of the laser while stripping all the other lasing modes, if any. Such a process of forcing a multimode laser to run in a single mode is typically very inefficient and difficult to achieve.
However the underlying physics of the laser device disclosed in instant application is different from those in the abovementioned patents, being based on the principle of imaging in a multi-mode interference (MMI) waveguide, whereby a beam profile at a given plane is reproduced periodically over an imaging distance as an optical wave propagates along the waveguide. Here, the cavity roundtrip length is chosen to match this imaging distance or a multiple thereof, while a single-mode waveguide provides a feedback source as well as a port for the optical output. A key characteristic is that the self-imaging distance is a simple function of the waveguide width, refractive index, and wavelength. This consideration does not arise in the prior art.
As a result, throughout the entire cavity except at the output, the beam is multimode and practically fills the entire waveguide. When it reaches the output plane, it collapses to a single-mode spot that is matched to the single-mode output waveguide. All of the modes are theoretically combining perfectly into a single mode at the output of the MMI waveguide. Also, the disclosed device design differs from prior art in that there is no need for a gradual tapered section—the transition from the multimode section to the single-mode waveguide can be abrupt.
Perhaps the prior art most relevant to instant disclosure is U.S. Pat. No. 6,768,758 in the name of Hamamoto assigned to NEC Incorporated. It utilizes a monolithic structure that is based on the MMI principle, starting with a single-mode beam, allowing laser gain to occur in a multimode beam, and then it subsequently reproducing a single mode beam. As the entire MMI is made of semiconductor, catastrophic optical damage (COD) at the single mode hot spot on the output facet is expected to limit the achievable output power. It therefore seems unlikely that this structure offers any significant benefit over current state of the art single mode lasers operating at the one watt level or below.
In contrast, instant application avoids creating a single mode “hot spot” on the facet of the semiconductor laser chip which is more sensitive to degradation in regions of high optical power density. The laser waveguide and the silica waveguide (coupled in close proximity) together constitute a MMI structure, so that the beam is multimode at their interface, in particular, the laser facet. The beam becomes single mode only inside the silica waveguide, which is better able to handle a high optical power density. As a result, the device is enabled to produce high-power single mode output power that should be comparable to what is achieved in multimode broad-area diodes, namely in the range of 5-10 watt or more. For example, at JDSU Inc. an optical power output of 14 W per device and over 70% electrical-to-optical conversion has been demonstrated in a broad-area diode.
Thus, an object of the present invention is to increase the stability and the output power of a conventional broad-area laser by lowering the peak power density within the semiconductor gain region. Broad-area lasers have not only much higher power but also higher inherent efficiency than traditional single-mode lasers and tapered oscillators since the lasing volume is more uniformly filled.
A further object of the present invention is to improve the manufacturability of a conventional broad-area laser through a robust multi-mode compound laser cavity design that is fully defined by the geometry and the refractive index profile of the MMI region, which is quite insensitive to slight macro-scale refractive index variations due to manufacturing variation or temperature. As the self-imaging position shifts linearly in the refractive index, the confocal beam parameter can be made relatively long.
Another object of the present invention is to use a geometry that can be engineered explicitly to avoid optical hot spots on the laser facet and inside the semiconductor laser chip.
Still a further object of the present invention is to leverage existing investment and expertise in broad-area laser technology.